We build a diminutive by adding the suffix -lein or -chen. For some nouns one is preferred over the other whereas with other nouns both may be used.
der Tisch | das Tischlein | das Tischchen
das Brot | - das Brötchen
der Bach | das Bächlein | -
In Hochdeutsch there is a tendency to prefer -chen whereas regionally the suffix corresponding the -lein-form (e.g. -li Swiss, -le Swabian) is used.
Apart from that, are there any rules that define which suffix to use?